Dr. Harrell, please bottom post rather than top post. It makes the thread easier to follow.
I was responding to your question, "Is a piecewise flat relationship more realistic than a linear one?" when I wrote, "Well, yeah, I would say so. If the relationship is U-shaped, say, then re-coding a continuous predictor variable into 5 categories, will provide a better fit. To which you are responding: Frank E Harrell Jr wrote: > > Definitely not. Categorizing into 5 levels takes 4 degrees of freedom > and still assumes a piecewise flat relationship. Regression splines > handle U-shapes as well as other smooth shapes, with typically < 4 d.f., > and they provide better fits. All I am saying is that the piecewise flat relationship would provide a better fit to a U-shaped relationship than a linear one would. I'm not arguing that splines wouldn't be better still. As for the degrees of freedom issue, in epidemiology, we often don't care, as our datasets often have tens of thousands of cases. -Jay . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
